The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document.
Non-contact sensors can be used to measure an object's distance and/or velocity. Non-contact sensors often emit energy signals such as one or more acoustic or electromagnetic signals toward an object. By analyzing the reflected signals, distance and velocity of the object may be determined. Examples of non-contact sensors include, for example, sonar, radar, laser, and UV devices.
Measuring fluid velocity and distance in partially filled open channels (for example, sewers, irrigation ditches, etc.) using radar is especially challenging. When measuring velocity of a flowing fluid, the signal must typically be emitted slantwise to the fluid surface, resulting in significant signal loss. The loss occurs because most of the incident energy glances off the fluid surface and continues propagating away from the sensor. Measuring distance in open channels may be very challenging when high accuracy is required (for example, millimeter accuracy at a maximum range of only a few meters). When monitoring open-channel flow, the measurement of distance is undertaken to determine the fluid level. With knowledge of the sensor position and channel geometry, fluid levels may be calculated from distance measurements. When measuring distance/level, the signal is typically emitted in a downward direction.
Recently, Frequency Modulated Continuous Wave (FMCW) radar systems have been designed which are capable of measuring fluid depth and velocity in open channels. Devices exist in which certain radar components are shared between a down-looking antenna for level measurement and a slantwise-oriented antenna for velocity measurement (see FIG. 1). A continuous wave (CW) signal for velocity is emitted in a slant wise fashion to the fluid being measured, and a frequency modulated (FMCW) signal for distance is emitted perpendicular to the fluid. A perpendicular signal is emitted to obtain a strong return signal so that the required distance accuracy is obtained. As a consequence, fluid level and velocity are measured at two distinct points.
When measuring fluid flow, volumetric flow rate can be calculated as the product of the fluid's cross-sectional area and average fluid velocity. For open channel flow (for example, sewers, conduits, etc.), the cross-sectional area may be calculated from the fluid level, given knowledge of the channel geometry. However, if velocity and level are not measured at the same point, computations of volumetric flow rate may be in error. For example, if conduit slope, or conduit cross-sectional area change between the two measurement points, or if a change in fluid level occurs as a result of some upstream or downstream condition, calculation of flow rate may be inaccurate. It is, therefore, desirable in non-contact flow sensors to measure distance and velocity at the same point.
To measure distance and velocity at the same point, both the velocity and distance signals must typically be emitted in a slant-wise fashion to the fluid, which results in most of the radar energy glancing off the fluid surface. Very little of the energy bounces back toward the detector. The resultant high loss adds significantly to the difficulty of making a valid distance measurement. Measurement becomes particularly difficult as the range gets shorter (in part as a result of leakage peaks and associated side lobes in the spectrum of the received signal).
Often, the signals used in FMCW ranging systems are emitted as “chirps.” Chirps are modulated signals that increase or decrease frequency with time. Individual linear chirps can be characterized by a starting frequency, a slope, and a time duration.
More general frequency modulations may be constructed by assembling a series of chirps end to end. Using a ramping phase locked loop or PLL, a diverse set of linear frequency modulated or LFM pulse trains can be created, each comprising a set of chirps. When each LFM chirp has a different starting frequency, the modulation is called a “stepped LFM sequence” Or a “stepped sequence.” The frequency steps may be chosen to be uniform or non-uniform in size.
When a modulated signal is emitted towards one or more targets by a radar or sonar transducer, reflections from the target(s) are synchronously demodulated to produce a signal called an “intermediate frequency signal”, or “IF signal”.
The demodulated IF signal may be digitized and analyzed via spectral analysis to determine information about the target, e.g., distance, velocity, etc. Spectral analysis refers to the analysis of the demodulated IF signal with respect to frequency, rather than time. This is often accomplished by calculating the signal's discrete Fourier transform (DFT), for example, via a fast Fourier transform algorithm or FFT. The output of a DFT-based spectral analysis is called a periodogram, or “power spectral density.” Depending upon its use, it may also be called a Doppler spectrum, velocity spectrum, or range spectrum.
Proper design of the modulation sequence is important to ensure correct sensor operation. If stepped LFM sequences are not designed correctly, the device may not function as intended. Proper design of stepped LFM sequences is necessary to preserve phase coherency, control the magnitude of unwanted and high amplitude side lobe signals, prevent aliasing, control noise, etc.
Stepped LFM sequences are also designed to accommodate spectral analysis (for example, FFT). Parameters of the FFT algorithm include frame size (or ‘number of bins’) and sampling rate. Frame size is the total number of samples used in the calculation and sampling rate corresponds to a frequency of data acquisition. Thus, a snapshot of the IF signal is gathered corresponding to a time duration equal to (frame size)/(sampling rate). This time duration is also called frame length. Ideally, the IF signal from a complete stepped LFM sequence will fit exactly into one FFT frame.
A radar instrument designed to measure a fluid level in open channels and sewers may require minimum range resolution of approximately a millimeter (mm) and maximum range of several meters (m). An FMCW ranging device designed for this range requires a very large FFT window that may be impractical, waste valuable computing time, and have large associated power consumption.
There exists a multi-scale analysis methodology that optimizes computing time and minimizes power consumption during FFT analysis for Doppler velocity measurements. The method includes several concurrent spectral analysis processes, each operating at a different sample rate and having a different window duration and frequency resolution. A decimation process is used to reduce the data rate between one analysis “scale” and the next. Because the same FFT length is used at each spectral analysis scale, a single FFT engine or subroutine may often be shared between all analysis scales.
Applying multi-scale analysis to an FMCW ranging device may significantly improve its precision and resolution while minimizing computation effort. However, conventional FMCW schemes cannot be implemented with multi-scale analysis, as a result of interference from high amplitude side lobes, aliasing, etc. during spectral analysis.